1. Field of the Invention
The present invention relates to a color filter substrate used in display devices such as liquid crystal displays, electroluminescent (EL) displays and plasma displays, and to a method of manufacturing the color filter substrate.
2. Description of the Related Art
Liquid display devices are being widely used in various electronic appliances because of their features: a small size, a small thickness, a low power consumption, and a light weight. In particular, active-matrix liquid crystal display devices having switching elements are being widely used for office automation machines such as personal computers, audio-visual appliances such as television sets, portable telephones, etc. In recent years, rapid progress has been made in improving the qualities of liquid display devices, e.g., increasing the size, improving the definition, increasing the effective pixel area ratio (aperture ratio) and improving the color purity.
The structure of an ordinary active-matrix liquid crystal display device will be described with reference to FIG. 8. FIG. 8 is a cross-sectional view of a liquid crystal display device.
As shown in FIG. 8, the liquid crystal display device 30 has an active matrix substrate 2 and a color filter substrate 4 opposed to each other, and a liquid crystal layer 6 disposed between these substrates. The liquid crystal display device 30 also has, in a substrate surface, a display region (effective display area) and a picture-frame region (non-display area) surrounding the display region.
The active-matrix substrate 2 is constituted by a transparent insulating substrate 8 made of glass for example, a gate bus line (not shown) for a scanning signal formed on the substrate 8, source bus lines 10 for a data signal, active elements (not shown) such as thin-film transistors (TFT) and transparent pixel electrodes 12. The gate bus lines, the source bus lines 10, the active elements and the pixel electrodes 12 are formed on the substrate 8. A plurality of the pixel electrodes 12 are arrayed in matrix form in the display region.
The color filter substrate 4 is constituted by a transparent insulating substrate 14 made of glass, for example, a color filter layer 22 including a red color filter 16, a green color filter 18 and a blue color filter 20, a light shielding layer 26 including a plurality of light shielding portions 24, and a counter electrode (not shown). The color filter layer 22, the light shielding layer 26 and the counter electrode are formed on the substrate 14. The red color filter 16, the green color filter 18 and the blue color filter 20 are provided in correspondence with the plurality of transparent pixel electrodes 12 provided on the active-matrix substrate 2 side. The light shielding portions 24 are disposed in gaps between the color filters and in the picture-frame region. Ordinarily, the counter electrode is provided on the surfaces of the light shielding layer 26 and the color filter layer 22 on the liquid crystal layer 6 side.
An example of a method of manufacturing the conventional color filter substrate 4 will be described.
In recent years, a dry film method has been used as a color filter substrate manufacturing method. The dry film method has the advantages of ensuring higher yield of materials and reducing the manufacturing cost in comparison with the spin costing method conventionally used. The dry film method also has the advantage of enabling the formation of a layer that is more uniform in thickness.
A method of manufacturing the conventional color filter substrate by using the dry film method will be described below with reference to FIGS. 9A to 9F.
A dry film is formed in such a manner that a photosensitive resin film is interposed between film supporting members such as polyethylene terephthalate (PET) films facing its two major surfaces. Four dry films in which red, blue, green and black pigments are respectively dispersed in photosensitive resin films are used. Typically, the photosensitive resin film is of a negative type.
First, the red dry film, for example, is adhered to the glass substrate 14 by being pressed against the same and the film supporting member is thereafter separated, thus transferring a red photosensitive resin film 16R onto the substrate 14, as shown in FIG. 9A. Ordinarily, this step is a thermal transfer step executed by heating the dry film. Subsequently, the transferred red photosensitive resin film 16R is exposed with a mask 32 disposed thereon and undergoes development. The red color filter 16 is thereby formed as shown in FIG. 9B.
Subsequently, the same step as that described above is performed by using, for example, the green dry film to form the green color filter 18 as shown in FIG. 9C. Further, the same step as that described above is performed by using the blue dry film to form the blue color filter 20 as shown in FIG. 9D. Thereby, the color filter layer 22 including red, green and blue color filters 16, 18 and 20 is formed.
After the formation of the color filter layer 22, the same method as that used for forming the color filter layer 22 is performed, that is, the black dry film is adhered to the glass substrate 14 by being pressed against the same to transfer a black photosensitive resin film 26R onto the substrate 14 as shown in FIG. 9E. Exposure (back exposure) is performed on the glass substrate 14 from the back surface side of the same. By this back exposure, the black photosensitive resin film 26R is exposed (in a self alignment manner, which means no use or alignment of separate masks is necessary) with the red, green and blue color filters 16, 18, and 20 being used as a mask. After exposure, development is performed to form the light shielding layer 26 having light shielding portions 24 disposed in the gaps between adjacent color filter portions and in the picture-frame region.
The manufacturing of the color filter substrate is thus completed.
In the above-described manufacturing method, since the light shielding layer 26 is formed in a self-alignment manner with the color filters each used as a mask (as shown in FIGS. 9E and 9F), no gap is formed between the color filters and the light shielding portions. Therefore, the above-described manufacturing method has the advantage of improving the yield. However, since the photosensitive resin film 26R of a negative type is used for the light shielding layer 26, the amount of exposure at the time of back exposure is limited to avoid setting of the black photosensitive resin film 26R formed on the color filters 16, 18, and 20. Therefore, the film thickness of the light shielding portions 24 is much less than the thickness of the color filters 16, 18, and 20, and a difference in level (indicated by X in FIG. 9F) occurs between the light shielding portions 24 and the color filters 16, 18, and 20.
In particular, if the film thickness of the color filters 16, 18, and 20 is increased in order to improve the display color purity in the case of making the color filter substrate by the above-described manufacturing method, the difference in level between the color filters 16, 18, and 20 and the light shielding portions 24 due to the film thickness difference between the color filters 16, 18, and 20 and the light shielding portions 24 is considerably large. If a difference in level exists in the surface of the substrate on the liquid crystal layer side, the alignment of liquid crystal molecules is disturbed by the difference in level which causes serious degradation in display quality.
The problem that a difference in level occurs between the color filters and the light shielding portions (that is, the upper surface of the color filter layer cannot be made flat) is not limited to the dry film method. The same problem also occurs in the case of use of a spin coating method and a slit coating method (a method of performing film forming by ejecting a film forming material through a rectangular slit and by moving the slit relative to the substrate while maintaining a predetermined gap between the slit and the substrate surface, which is sometimes called a slot coating method or a dye coating method) and other film forming methods. Various attempts have been made to solve these problems.
For example, Japanese Laid-Open Patent Publication No. 7-120613 discloses a method of reducing the difference in level between color filters and light shielding portions by performing a development step two times. More specifically, in this method, a light shielding film (black photosensitive resin film) is formed so as to cover a color filter layer and is exposed through a photomask from the upper surface side of the substrate and development is performed to form light shielding portions. When the light shielding portions are formed, internal portions of the light shielding portions are not substantially set. Also, due to an alignment margin (misalignment), the light shielding portions are formed so as to overlap end portions of the color filters. Subsequently, the shielding portions are set on the substrate side by performing back exposure. Thereafter, the second exposure is performed to remove portions (projections) formed on the color filters in the light shielding portions. According to the description of this method, the projections are removed by the second development step to flatten the upper surface of the color filter layer. In the method described in Japanese Laid-Open Patent Publication No. 7-120613, a dry film method is used to form the light shielding layer.
Japanese Laid-Open Patent Publication No. 8-248409 discloses a method of flattening the upper surface of a color filter layer in such a manner that a base layer having a flat surface is formed on a substrate by combining light shielding portions and transparent resin portions, and the color filter layer is formed on the base layer. The transparent resin portions must transmit visible light in this method. More specifically, in this manufacturing method, light shielding portions are formed in correspondence with the peripheries of pixels, transparent resin portions having the same thickness as that of the light shielding portions are thereafter formed on regions where the light shielding portions are not formed. A base layer formed of the light shielding portions and the transparent resin portions is formed on the substrate. A color filter layer is formed on this base layer. According to the description of this method, the upper surface of the color filter layer is flattened since the color filter layer is formed on the flat base layer. In the method described in Japanese Laid-Open Patent Publication No. 8-248409, a spin coating method is used to form the light shielding portions.
However, there is a problem that the upper surface of the color filter layer cannot be sufficiently flattened by either of the methods described in the above-described patent documents.
In the case of the method described in Japanese Laid-Open Patent Publication No. 7-120613, the light shielding film is exposed by using a photomask and, therefore, misalignment can occur in the pattern of the light shielding portions depending on the accuracy of the exposure apparatus, resulting in failure to form the light shielding portions in the desired configuration. In such a case, a gap maybe formed between the color filters and the light shielding portions in some location in the light shielding layer or a light shielding portion having a width exceeding the alignment margin may be formed on a color filter end portion, resulting in the formation of a projection or a recess in the upper surface of the color filter layer even after the second development step.
In the case of the method described in Japanese Laid-Open Patent Publication No. 8-248409, the color filter layer is formed on the flat base layer but a gap may be formed between the color filters to form a recess or the color filters may overlap one another to form a projection if misalignment occurs in patterning of the color filters.